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See my comment here [slashdot.org] where I wonder if maybe we're getting way too excited about dark matter without having any material reason (other than "this is the only explanation that fits our current expectations of gravity") to believe it actually exists.

Based on the moderation that followed, I would say that "some people" don't like it when popular theories get questioned. Which just goes to show you--once a scientific "fact" has been established, our attachment to it becomes as dogmatic as any theological notion...

There seems to be, as you put it, a "dogmatic" belief, often from undergrads (I'm guessing), that their now current understanding of physics is "right", and that any questioning of dark matter is an excuse to call the qestioner ignorant.

I've asked numerous times why I should think dark matter is anything other than a mildly promising theory.

I think that overall this is a good point, but I've seen many people who make their living doing hard science who, once they open their mouths, start putting their personal credibility where it does not yet belong.

Although I don't do it for a living, I'm dedicated to science and it's progress and I have a real love for both the process and the results. But I'm afraid that one of the biggest factors that has made science vulnerable to inroads by fundamentalists is that scientists have, of late, embrace three (admittedly hastily constructed) levels of credibility on scientific subjects:

1) We very strongly believe this is true because it has been repeatedly verified through controlled experimentation.

2) We very strongly believe this is true because it can be strongly inferred from existing verified data.

3) We understand that we don't have all the facts, but we are critical thinkers for a living and our theories are worthier than your theories.

Number 1 is where scientists should be, but in debates, articles, and various other discussions on the battle between religion a science I have seen prominent and credible scientists arguing 2 and sadly, much more often 3. I understand it, but many scientist should reign themselves in.

Understand, I'm not saying don't fight, just that we should fight from our position of greatest strength, which is being "fundamentalist" about the scientific method. If we can teach kids, or anyone, how it works and why we're devoted to it, all the while showing by example how to be scientific in thought, then we win.
There's plenty of room for religion in the world even with hard science, and there's plenty of amazement and wonder to be had in science too. I just don't want to see scientists try to expand their own role in human exploration way beyond the data.

Within the jargon of science, "true" means "useful and predictive". Scientists sometimes forget that they have a nonstandard meaning for "true", especially if speaking casually. It's worth noting that this jaron definition of "true" is more useful and predictive than the standard meaning.:)

I think this demonstrates a fundamental misunderstanding of scientific research. Any real scientist will tell you they're wrong more than they're right.

You start with an observation, come up with a reasonable hypothesis to explain it, then test it.

Eventually your hypothesis fails at some level. So based on your observation, you create another reasonable hypothesis....

That's scientific progress. Each step along the way we learn more. And often, we get led down the wrong path, for any one of many reasons -most are not evil.

As a scientist, I can tell how I think many things work. Of course that leads to the question, "Don't you know for sure?" from a non-scientific public who wants to know that the levy will hold or the vaccine will protect them from disease and not cause it.

No, I don't know for sure. But that's not what anybody wants to hear. And that's not what anybody will report in the press. That's not what politicians base decisions on. The overwhelming majority of times you see science misused as you stated above its by companies/politicians/people taking scientific data and theory and restating it as scientific fact. Its rarely the scientist doing the study who says such things.

Since so many have had to put up with "fundies," and the "just a theory" camp.

The problem, as I see it, is often that those who question the theories don't have anything better to back them up -- they're just presented with skepticism or an alternative answer that has nothing to do with physics.

Of course, I think there's more room for that in astrophysics, given the focus on math and proofs rather than testing (due to rather obvious logistics). A new mathematical proof can come out that completely changes how people view space (or, heck, an appropriate use of an old mathematical proof, as the article shows).

But I can understand why some people would be a trifle edgy nowadays. I'm not saying that you provoked the argument, as I've dealt with scientists (heck, I live with one and hang out with her friends), but I have to ask -- when you said it was a 'mildly promising theory,' did you present an alternative opinion? One thing I learned is that scientists really dislike people saying "I don't believe that" or "I think that theory is wrong" but then don't offer what they DO believe in that's based on science. After all, that doesn't accomplish anything -- it just states a claim of belief, which isn't science.

But I don't think I need to explain why an "accepted theory" will have people assume that it's accurate and "true" and be reluctant to drop it just at some new information or test or mathematical proof. That older theory generally has plenty of evidence to back it up -- the new theory has none. So people will look at the new theory, run through the math or tests on their own, and confirm, therefore changing the general understanding. That's how science works. The reluctance to accept just any new information without seeing a lot more proof is one of the reasons science tends to add to a base of knowledge, rather than jumping down any old path.

Dark Matter is far from an accepted Hypothesis, yet seemingly intelligent people defend it on the basis that it's the best thing going.

That's just stupid. Science isn't about being right, or falling into lockstep with "accepted theories", it is about continually asking questions.

My question about dark matter has always been "Why is it more acceptable to make up a new type of matter, rather than deal with the idea that the fundamental forces may work differently than is believed?"

Why is one SO MUCH better than the other? There is precedent for both possibilities.

I think the reason is that in seeking for more fundamental theories in particle physics we get theories which imply there are some particles we don't know yet. Therefore it's not too unlikely that there is a form of matter we don't know yet, and there's no known reason that it may not be enough to be relevant in large scale structures. Therefore, introducing dark matter means introducing something which we might well have to introduce anyway. Changing the law of gravitation means doing another, independent

My question about dark matter has always been "Why is it more acceptable to make up a new type of matter, rather than deal with the idea that the fundamental forces may work differently than is believed?"

Well, because there was no theoretical framework to explain the data without the use of darkmatter. Let's face it, the whole darkmatter hypothesis is extremely ad-hoc, a fudge factor added into galactic rotation calulations to make them fit to what was expected. The outcome was a predicition that darkmatter must exist.

Now, there is nothing particularly unscientific about this. Go take a look at particle physics where all kinds of particles were predicted to exist, and as a result many particle physicists went out looking for these particles. When they were found, this confirmed the theory, when the particles were not found, they continued to look, or they revised the theory.

The same kind of thing happened here. People have been looking for darkmatter for quite some time, however, it appears that a revision to the models used to predict galactic rotation curves *and* galactic clustering is what's needed.

Why was the existance of darkmatter more "acceptable?" 1) Basically, because it was a prediction that fit the models. That's something that scientists like a lot, it gives the experimentalists something to really sink their teeth into. And 2) there was no way to predict that a change in the theory was needed without having already developed a theoretical framework that could explain galactic rotation curves without the need for darkmatter.

As an astronomer, I would say that you're not wrong to ask your question, however, without having any idea of how our theory might need to be changed, it's kind of a pointless question. And in this case, it sounds like we really don't need to change our theory at all, it turns out that the range of validity of Newtonian gravity is a lot smaller than we thought.

I think the bigger question in my mind is why hadn't someone tried to do this before now? In some sense, it's one of those things that just kind of surprises you, because all of a sudden you realize that *everyone* has been operating under the same false assumption about Newtonian gravity, and then you wonder why nobody thought to check that out.

Of course, this all assumes that this new model using relativity is correct... It probably is, but I think it does need to under go the usual scrutiny just to be sure.

I think the bigger question in my mind is why hadn't someone tried to do this before now?

Science isn't truth, and it isn't fact. It's a process that, over time, results in a gradual and constant tendency towards truth.

If you get into a debate with religious folk about "creationism" versus "evolution", one of the tactics almost invariably tried is to disprove some facet or other of evolution through some form of deductive reasoning. The basic idea is to prove that Science is somehow wrong, and then assume that creationism wins by default once that's done.

It's easy to see the fallacy: disproving evolution (even if they can) doesn't prove creationism.

But, scientific theory is always undergoing review and clarification. Newtonian gravity works, in limited scopes. It was revised and improved with relativity theory, which is itself being revised and improved today with multidimensional, superstring theory. It's this recursive process of deduction, testing, and review that advances science.

We should be ecstatic! Despite our incredible efforts to find it, we've uncovered NO evidence that this has ever happened before in the multi-billion year history of the universe!

People are stupid, and we have to acknowledge that. Our intellect barely rises above our other urges, the urge towards sex, the blindnesses caused by our tendency to suspend reason (A.K.A. "Faith") and follow the leader 'cause it's easy. And, truly fresh/new approaches to problems are rare, and hard to find. Most any "new" thought is merely an extension of a previous thought. We're creatures of habit. But, so long as we continue to try, so long as we continue to be willing to challenge our assumptions, and take the time to do so when somebody DOES come up with something new, then the process of Science progresses, and life continues to get better.

Schools today don't teach science. They teach "facts", like "water vapor absorbs light, but absorbes blue light the least, and thus makes the sky blue". They don't ever teach the method of science, the passion of science, beyond making you recite the "gather facts, form hypothesis, test hypothesis, draw conclusion" which is only minimally how science works.

Children are BORN scientists. As they explore with their hands, and their minds, the world around them, they perform hundreds of experiments a day, every day. Where do you find frogs? What bug is making that buzzing noise? What happens if you clap your hands near a grasshopper? How many blocks can I stack up before they fall over?

So, what do we do? We lock them up in a sterile environment, where they're told not to question the teacher, and never to talk to the kids next to them. We prevent their natural curiousity, and instead, browbeat them into performing tricks like a circus animal. The apathy of the schoolchild is both detrimental and obvious.

And after that's done, after the child's natural, scientific curiousity has been conquered, that's when we introduce the wonders of science in the most boring, unimaginably unflattering way possible, by forcing him/her to regurgitate "facts" that they'd be ridiculed to question.

The real wonder? How does science advance at all in the face of this educational travesty?

It's pretty obvious that scientific curiosity is built into the very fiber of humanity, or how else could still be advancing despite our incredibly expensive social efforts to prevent it?

I wanted to suggest a couple ideas. First, dark matter is a well-favored theory because there is a lot of evidence that supports it. Galactic rotation speeds is one important piece of evidence, but I also think that gravitational lensing provides strong evidence -- which may also be explained by the GR work done in this paper. I don't know but it seems possible. I'm not an astrophysicist, and couldn't (or didn't waht to) follow all the details of the paper. Fluctuations in the cosmic microwave background is another piece of evidence for (cold) dark matter, though it gets complicated here. I don't think that the CMB directly requires dark matter, but dark matter models have been very successful here. I'm out of touch with recent CMB and cosmological accounting developments.

Anyway, the point is that the theory of dark matter kills a lot of birds with one stone. So it's very attractive from that point of view. And there are literaly dozens of yet-untested theories that can explain dark matter as exotic particles, compact massive objects, and so on. Many of these theories have been either disproved or damaged by careful experiments, but by no means all of them. So the existence of dark matter doesn't seem all that far-fetched either.

A second point is that a lot of this discussion has to do with scientific theories being "falsifiable", a term very much at the heart of the debate on creationism being taught in science classes. I don't think many people appreciate what the term means. Science cannot prove a theory to be true. You can only prove it to be false. Take "Newton's laws" example. It took somewhere around 250 years to prove those wrong, and relativity suffered a lot of ridicule from scientists still unwilling to let go of them.

Well, even though there's no way to really prove a theory to be correct, a theorist still has to start somewhere -- put their faith in some basic assumptions before any progress can be made. The choice of these assumptions is mostly a matter of taste, and a little bit of cleverness -- how can you keep your set of assumptions as small and palatable as possible?

General relativity is a really nice theory, and has stood up to a great deal of testing. It is thought to break down only on small scales far beyond our experimental reach, and there is no compelling reason to suspect its accuracy on even cosmically large distance scales. So it makes for a nice starting assumption for astrophysics. I guess the point of this paper is that some details have been forgotten about when modeling galactic rotation. It was thought that because of the small speeds involved, and weak gravitational field, that newtonian gravity (which is much easier to deal with computationally) was a perfectly good approximation. The author of this paper realized why it was not, and points this out.

I can only imagine that, if the math is correct, this will have a huge impact on the astrophysics community. For example, they mention why newtonian gravity works so well for our solar system still, but I'm not sure any more that it would work well for cloud collapse and star formation models. If it affects these models, it will probably also affect cosmologists modeling the evolution of structure.

Hi --- I'll give a shot at answering (I'm an astrophysicist, if that matters)

I'm not quite sure what you mean by "Dark Matter is far from an accepted Hypothesis". It is certainly not far-fetched to imagine that there is some quantity of matter, perhaps substantial, that does not "glow" like stars do. This is why it is "dark". The original problem was one of galactic rotation curves --- matter in the outskirts of galaxies rotated around the center in a fashion exactly mimicing what it would do if there was a spherical distribution of matter extending beyond the glow of the visible disk. The hypothesis that there was just such a distribution that we cannot see is not so far-fetched. It has been admittedly difficult to identify the "conventional" bodies that could be responsible for the lion's share of such a halo. Upper limits on the numbers of brown dwarfs, Jupiter-sized objects, and small black holes have shown that no one of these are primarily responsible. Still the search continues, as it would in any good scientific theory. Any of these possibilities are seen as a simpler approach than modifying our most basic models of gravitational behavior, especially when there is no similar pattern of deviation from known laws on different scales. And, as shown by the follow-up paper in the archives, there is a real possibility that the authors have made an honest mistake.

I have often mentioned my disbelief in common astronomical theories to my fellow students at the Niels Bohr Institute here in Copenhagen, and not once have I been meet with an attitude like the one you describe. (For instance I don't believe such a thing as a GR-black hole actually exists..)

In my oppinion your fellow students are seriously lacking in their scientifical education if they are unable to accept that alternative theories should be considered seriously but critically.

"because QM is true and that's just the way it is.. "I find this funny because I too see this a lot. It must come from trying to teach the subject. When pressed, however, I've had people fall back to "OK, it could all be wrong, but *you* have to proposed a better quantum theory of measurement first". So I think even Quantum(tm) alternatives may be considered seriously but critically, it's just that you have to propose a very broad replacement theory, beyond what could easily be expressed in English.

In my experience, undergraduate science students, at least in the US, are usually of the belief that they are being taught "facts". Maybe in an introductory class more emphasis is placed on the unknowns, but as they move into their specialties all but the most controversial or speculative ideas are presented as facts.

Generally as they move into graduate studies there is more emphasis on the quest for knowledge as opposed to the memorizing and understanding of facts.

As one of my professors said my first year of graduate school, "You're graduate students now...you're allowed to have opinions."

IMO, all science degrees should include a class in Philosophy of Science. Most undergraduate students I've talked to about this idea say something along the lines of "Philosophy has nothing to do with science."

Since they currently have the upper hand, they are determined not to give any ground, the mere mention that evolution has some competing theories is completely unacceptable, it must be taught as absolute fact with no questioning allowed

There aren't any competing scientific theories outside of evolution so I'm not sure what else a biology class could teach. Obviously, evolution is not cut dried, it's science, its a living thing that is being updated constantly. That's why we have journals.

Maybe you're thinking of Creationism/ID? I guess you could hold it up as pseudo science (what not to do) but that's more pertinent to a philosophy class.

Please, global warming is a fact, the man-made greenhouse effect is the theory that is being questioned. Please keep them separate. When you question global warming you have to back it up with proof that the temperature measurements from the past century are wrong.

Which just goes to show you--once a scientific "fact" has been established, our attachment to it becomes as dogmatic as any theological notion...

Perhaps for some people, but not for the overall scientific community. This article being the most obvious example. And I need not note the difficulty one would encounter trying to debunk a theological notion...

I've been suggesting for years that "dark matter" is an unnecessary idea which only exists as a transitional kludge until we can uncover some more fundamental error in the theory of gravity, like planetary epicycles or what not. I have made this suggestion both on the internet and in person to some people I hang around with from my college's physics department.

While generally people have not agreed with me, I have never encountered what I would call "dogmatic" resistence; I never felt that people were upset at my suggestion or disrespected my opinion that this was a possibility.

Perhaps the reason why you have met with poor results expressing the same idea have more to do with the way in which you expressed the idea?

I find a lot of people seem to believe that if people disagree with them, it is automatically because of dogmatic resistence. Not necessarily, maybe it's just because you've not made your case very well, or because there are other factors to the discussion you aren't considering (for example, that asking a physicist to abandon the idea of dark matter would-- in the absence of a better explanation for anomolies in gravitational theory-- effectively require them to accept the idea that the galaxy is the wrong shape for no reason whatsoever).

"'Dark matter' is simply all the mass of the universe that doesn't emit detectable radiation."

Actually, the whole "dark matter" theory would make a lot more sense if this were true. Unfortunately, in order to fit our observations, dark matter also must not reflect any radiation, either. Furthermore, being theoretically non-baryonic [wikipedia.org], it actually allows radiation to pass through it, rendering it invisible. So it's a whole lot of undetectable, unverifiable mass which, save a few gravitic anomolies, we would never have thought up.

I believe the solution known as "dark matter" is an approach known as multiplying entities. Same as the "aether" light was thought to travel through--even though I have been criticized for making the comparison.

Not as long as it allows one to make correct predictions (at least within a certain domain).

Just to remind, at the beginning of the last century, many people considered atoms and their particles a mere abstraction, not necessarily representing the way things actually work, but rather serving as a model close enough to do meaningful calculations. You could say the same about dark matter here.

I'm so happy this main reason for Dark Matter finally got explained with standard physics.

I didn't quite buy the whole idea of Dark Matter, it wasn't scientific enough. We took a stab and said that there were phantom particles that we couldn't see and they were causing our observations to be different from what they should be. It just seems like we assembled a mythos. DarkMatter, the God of the Slow Galactic Turn, floats unseen at the edge of all galaxies. 90% of all matter is dark matter, and no you cannot see any of it (short of one lensing effect from an unknown object). So verifying this theory is next to impossible. And after a while we took the leap to say that we were correct. Even though we just invented stuff to 'fix' the flawed equations. Not that we can't guess right the first time, but just inventing a solution with no basis shouldn't hit the nail on the head.

I think the comparison between Luminiferous Aether and Dark Matter is one of the most prudent ones I've heard in a long while. Making something up to force your data to fit is a pretty bad idea. We can't be wrong. There's something that we cannot see that exists (does some calculations)... here; that makes the data roughly fit. It might as well have been the law of invisible elves of slow rotation.

And yes, if by some odd happening this gets peer reviewed dead... I still believe everything I said.

I think the comparison between Luminiferous Aether and Dark Matter is one of the most prudent ones I've heard in a long while. Making something up to force your data to fit is a pretty bad idea. We can't be wrong.

Except this has happened many times in physics with successfull results. The neutrino was a predicted particle that interacts weakly with normal matter. It was predicted in 1931 by Wolfgang Pauli to explain the result of experiments measuring beta decay. The particle wasn't actually detected until 1956. Does this mean Dark Matter must exist? Obviously no, and if this new calculation pans out it most likely doesn't exist. But that doesn't mean that proposing something new to fit your data is bad science. It obviously is good science, just make sure your prediction can be falsified.

I've never trusted this whole bandwagon regarding both dark matter and dark energy. It just seems like all of a sudden, with very little to actually invoke by way of proper observational evidence, everybody got on the dark matter/energy bandwagon and we were off to the races with it, despite continued objections from various and sundry quarters to the effect that we really do not have any actual evidence for any of this stuff.

From what I can recall, the whole idea of "dark matter" started with the observation that the galaxies are spinning too fast that the gravity of their visible matter would be able to keep them together. This led to the conclusion that one of the followign must be wrong, in descending order of propability:

The observations are incorrect.

The calculations are incorrect.

The underlaying theories (of gravity) are incorrect.

Galaxies are not stable structures, almost all the visible stars just happen to be arranged in galaxy-shaped formations at this point of history; in other words, this moment is special in the history of the universe.

The scientists started with the most likely alternative, namely by assuming that their observations were incorrect. There's two variables being observed here: the speed of galaxy's rotation and its mass. The speed of rotation is pretty easy to figure out: you simply need to compare the red and blue shifts in the spectrum of the stars of the galaxy.

On the other hand, mass is impossible to detect directly; you can only calculate its presence by the gravitational effects it causes. Assuming that the galaxy is a stable structure, you can calculate the mass once you know the rotation speed. After this calculation was done, the result was much larger than one would expect from the amount of visible stars. The obvious conclusion was that the majority of the mass was in a form that didn't show up in any way but through its gravity. The scientists accidentally added some overdramatization to the concoction, and thus the term "Dark Matter" was born.

Instead, we have a bunch of "unexplained" things that then get "explained" using dark matter/energy.

No, we had a bunch of unexplained (without quotation marks) things that then got explained (without quotation marks) using dark matter/energy. The explanation might very well turn out to be incorrect, but putting quotation marks on words does neither make any point nor show the hypothesis incorrect.

Sounds more like religion than science if you ask me.

Good thing that no one asked you, then, since this seems like a prime example of how science is made: you notice an unexpected phenomenon and try to find an explanation.

So it's nice to see some substantial cracking in the edifice, and I'll be quite pleased if the whole dubious enterprise comes crashing down and we revert to science that's either grounded on more substantial claims or is man enough to admit it doesn't know.

It is rather difficult to think up a more substantial base than the apparent conflict between observed reality and theoretical predictions that lead to the hypothesis of dark matter. And sitting around saying "I don't know" isn't a scientists job, trying to find out is. Being humans, they sometimes get things wrong; after all, certainty is not the domain of mere mortals. However, they are men enough to risk being wrong, even if someone will mock them on Slashdot when there is a suspicion of them being so.

The paper concerned doesn't "disprove" the existence of dark matter any more than conventional wisdom regarded it as "proved" in the first place.Sensationalist headline and non-TFA-reading posters aside, the paper merely shows that there is an interpretation of general relativity that someone's only just discovered that eliminates the need for dark matter to explain one type of observation where the theory didn't quite fit reality. There are several other scenarios where Dark Matter is still thought to be

This has always struck me as an anthropocentric, 'faith-based' element in modern physics. Why should the universe be simple and elegant? Because it's 'beautiful'? Because we don't like doing hard math problems?

I'm not against it, but it seems to be taken on faith that the universe should be simple and elegant. So far the track record is pretty good, but that doesn't mean that it's a scientific belief. Don't get me wrong, I'm not against a simple-and-elegant universe; I just haven't heard any scientific ex

In physics, the simpler answers tend to be the correct ones. Note that General Relativity is far more complex than Newtonian physics, though it's universally accepted that the former is the correct theiry. Newtonian physics is regarded as a reasonable approximation most times. It's why it's still taught in schools (you don't need to take relativity into account when measuring the momentum of a car going down the freeway, for example).

A galaxy is modeled as a stationary axially symmetric pressure-free fluid in general relativity. For the weak gravitational fields under consideration, the field equations and the equations of motion ultimately lead to one linear and one nonlinear equation relating the angular velocity to the fluid density.

That's really interesting. It makes sense to model a galaxy as a fluid on a very large scale. After all, gravity is a relatively weak force. I haven't gone through the paper, but if their math is right, since the assumption is relatively benign, this seems like it would be experimentally verified.

Since the model assumes that a galaxy is a fluid (on a large scale), the model would predict fluid-like phenomena. What I wonder is if there is a galactic analogue to solitary waves. How would these manifest? (A friend wrote his thesis on solitons)

This sort of model is not new, however there is a big twist as the range of forces in normal fluid mechanics is relatively much shorter (Lennard-Jones drops off at r^6) while gravity is a r^2 force. This makes the modelling a lot more complex.CC Lin has been using this approach to model the evolution of the spiral structure of galaxies for some time (mid 70's or earlier).

Yes. The whole idea of dark matter was a kludge to explain why they weren't seeing what they expected. I always figured that they were interpretting things incorrectly, or measuring incorrectly. For one thing, it's mainly based on how much ordinary matter we expect in "empty space", or the space between stars. Recent discoveries suggest that there may be a lot more matter in the Oort cloud [wikipedia.org] than we originally thought. (Although still pretty small, compared to the sun -- but I think we're still under-estimati

Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.

I noticed you were referring to an article on arXiv.org [crookedtimber.org]. While it may certainly be true, these articles have not been peer reviewed by a scientific journal. Also note that this author appears to have only the single article on the site (which may or may not mean anything - draw your own conclusions).

I think arXiv.org is a great idea - a way for physicists to communicate ideas informally before going through the hassle of getting them published. It's still best to take it all with a grain of salt, as p

I read the article and I fail to see how this criticism holds water. The article under discussion proposes a thin disk of non-luminous (as opposed to "Dark") matter on the same plane as the galaxy, in roughly the same distribution as the luminous matter. Such a disk could be made of dust or other particles, would be an order of magnitude smaller than the proposed Dark Matter, and would potentially be difficult to observe. One wonders whether the refuting author finds Dark Matter more or less "physical" than a thin disk of real matter.

At first, maybe. When the speed at which galaxies rotate wasn't as expected, one legetimate proposal is that there's enough "dark matter" to make up the difference. After all, astronomy is inherently limited to what we can "see", and matter like dust clouds that obscure what we can see. The universe could be full of matter that doesn't interact much with light, and we'd only find out about it when we started measuring large-scale gravitic effects. Not really an appeal to magic, more of a "this makes the

The neutrino, when originally discovered, was discovered because something was missing. Particle collisions were seemingly violating the conservation of energy and momentum. Postulating the existence of an unknown, massless or nearly massless particle that interacted only weakly solved that problem.

Only later was the neutrino discovered.

Unanswered questions, very specific unanswered questions (we need *something* to do *this*) often do lead to new discoveries in science.

I'm not saying that dark matter necessarily has to exist, but the galaxy and cluster gravitational dispersion evidence were strong indicators that there had to be more gravity there. Postuatling that we weren't seeing all the mass was a very reasonable postulate. Now there are lots of other reasons (e.g. CMB, large scale structure evolution) to suspect it's there. And, possibly, in the next decade, we will finally identify the dark matter particle in the lab. We'll see.

Perhaps it is a WYSIWYG universe, we just don't understand how to properly see what we see.

This may also be a cautionary tale about the use of linear models (Newtonian gravity) versus nonlinear ones -- interactions among masses distort the solution. If one assumes the wrong things and gets an answer that doesn't fit the observations, perhaps its time to change the assumptions, not add unseen dark matter, epicycles, etc.

...is going to become the major worry. Data from supernovae distance measurements indicate that the Universe has been expanding for some time already. That means that there has to exist a sort of anti-gravity (called dark energy by astrophysicists). Now, that is hard to explain by conventional means (although it is possible), and may involve either a "beyond Einstein" type of theory (e.g., an improved general relativity) or some exotic form of energy (or both). So, although general relativity alone might account for the rotational curves of galaxies, it does not account for the large-scale properties of the universe.

The poster title is misleading, the paper still leaves a place for dark matter, but on very smaller amounts and far from the halo. So, this matter could easily be barionic (paper's conclusion).

What is really interesting is that the third galaxy didn't fit the model as well as the others. It may be because of the inacuracy of the calculations (is the inacuracy measurable? The paper should have said that) or because there is something different on this one, maybe a smaller concentration of dark matter near the center.

Really?! I'm interessted in astronomy and physics at a hobbyist level, and have always assumed that the simulations of gravity and galaxy formation was done with relativistic mathematics. Instead they have used approximations using newtonian theories? WTF? No wonder they came out wrong!

I can live with newtonian approximations on a solar system level, but doing cosmology on the scale of galaxies, the age of the universe it self and so forth they really should have used the sharpest tool in the tool box.

If I had the knowledge and the machine power to do simulations my self I would've done so, but I don't so I trusted the astronomers. They really shuldn't have taken the shortcuts, escpecially after their scientific profgress went boink and they started devicing exotic new models just to cover up their seemingly faulty theories! Shouldn't they have done a simmulation without the approximations just to evaluate how good their approximations was?

As another poster (here [slashdot.org])has already pointed out, other physicists have since worked through the algebra of this paper and found it lacking.

I'm told there is a mistake in the general relativistic metric used in the paper. Basically, a small error left them modeling the wrong situation. The situation they actually studied was one with an axially symmetric cloud of self-gravitating gas (the galaxy) AND a thin, heavy disk. The thin, heavy disk screws things up and produces the effect they observe.

What does this imply for cosmology and particle physics, both of which have been worrying about other aspects of dark matter?

I think it implies that we can stop chasing for something that probably doesn't exist, and get about the business of finding out what's really going on out there.

Maybe it's just me, but the first time I heard about dark matter and how it "must be out there" because it makes the calculations add up nicely...first thing I thought of was the ether. For a long time we needed an ether to explain radio waves, light propogation, etc. Turns out the truth of the matter is something totally other. And it's a far more facinating other, IMHO.

I'm guessing that hundreds of years from now, physics students will be reading about dark matter and chuckling. Same way we do today when we read about the luminiferous ether [wikipedia.org].

Science began by making apparently unrelated observations, and later filled in the gaps to create unifying theories. Nature's apparent horror of a vacuum, and a whole lot of other phenomena, are explained by the pressure in a fluid acting equally in all directions. Many phenomena suddenly made sense when it was discovered that matter attracts other matter.

Now, we still have a few gaps, including that small things appear not to behave the same way as big things. No doubt, if we can quantify the differences* -- or explain why that would be impossible -- we can take a stab at a single Grand Unifying Theory which would underpin all of Physics.

It's also possible that there could be another possible set of laws of physics which would be mutually consistent, even consistent with the G.U.T., just contrary to all our observations. If there existed a parallel universe which obeyed this set of laws, one of four things could happen:

It would collapse to a single point in our space

A single point of space in that universe would be bigger than the whole of this universe

It would exist for only a brief instant of our time

A single instant of time in that universe would last longer than the lifetime of our universe

Of course, it's also possible {but extremely unlikely} that there is no Grand Unifying Theory, just a supreme being with a sick sense of humour who keeps changing the rules slightly every time we get close to discovering what they are.....

* Canonical example of difference between quantum and classical phenomena: Why can't a chair just spontaneously shift position? My own take is that quantum wave functions do exist in large systems, but "quantum" phenomena are not generally observed because the waves are not coherent {just as you don't see interference fringes where the light from two candles falls on the same surface}.

Consider the chair as a fundamental particle. It can be described in terms of its mass, as a particle, or of its wavelength, as a wave. How far you can expect tunneling in a chair can be observed is a function of its wavelength, and for an object as massive as a chair its wavelength is terribly terribly small...

Seems like somewhere in the translation from this preprint to the popular press, this turned into "no dark matter after all." Let me put this in context.

There are two problems in astrophysics in which dark matter is invoked as a possible explanation. One is the "galaxy problem": galactic rotation curves imply a distribution of matter different that you would infer from looking at the luminous matter. The other is the "cosmological problem": observation of redshift vs. distance, and of the cosmic microwave background, and other similar measurements imply a total mass density in the Universe different from what you would infer from looking at the luminous matter. Each of these problems can be explained with dark matter (e.g. some kind of extremely massive particles that only participate in the weak interaction, not yet observed). Sadly, the properties of the dark matter needed to solve these two problems are not necessarily the same.

This paper claims to eliminate the need for dark matter to solve the galaxy problem, but does not address the cosmology problem.

I'm not in this field anymore, but I spent 4-5 years in quantum gravity, black hole astrophysics, and inflationary cosmology. Summarizing my reactions to other comments in this thread:

First, I will say that I have not gone through this new paper in detail. I'm skeptical at a gut level that their results seem to depend on general relativity, because GR should not be relevant on the scale of galactic rotation curves: there is good reason why all the calculations ignore GR. It makes me think that there is a flaw in their calculation, and indeed another poster referred to a potential rebuttal of their GR analysis.

Second, as yet another poster mentioned, galactic rotation curves are just ONE evidence for dark matter. We have evidence from the aforementioned stellar orbits in galaxies, plus the motions of satellite dwarf galaxies, gravitational lensing, measurements of galactic gas temperatures (depends on the local gravitational neighborhood), anisotropies in the CMBR, the rate and structure of large-scale cosmological structure formation, etc.

(There are also a bunch of theoretical reasons to believe that dark matter particles could exist purely on the basis of particle physics, even if you ignore the astrophysical evidence; see axions, supersymmetry, etc.)

It's not surprising to come up with an alternative that can explain ONE of these phenomena. In fact, there is already another alternative that can also explain galactic rotation curves: MOND (MOdified Newtonian Dynamics), an alterating of Newton's laws of gravity. (There is a relativistic extension by Bekenstein, although it's currently even more ad hoc than dark matter appears to be.)

The problem is coming up with explanations for ALL of these phenomena. Dark matter is the only theory that has been able to do so, and it's not for lack of trying. Contrary to popular Slashdot groupthink, scientists are not in love with coming up with the most absurd and exotic possibilities they can. Most astronomers hated dark matter. For decades. I even know one who only came around to it a few years ago. It's simply that dark matter works, and everything else people tried to propose in its place didn't. As Carl Sagan said, "No physicist started out impatient with commensense notions, eager to replace them with some mathematical abstraction... Instead, they began, as we all do, with comfortable, standard, commonplace notions. The trouble is that Nature does not comply."

Now, this is not to say that dark matter is the end-all, unassailable dogma. It's possible there are alternatives, including modifications to gravity. I like to compare it to the discovery of Neptune and the perihelion precession of Mercury. People say that it's ad hoc to postulate unseen matter to explain gravitational anomalies. But that's precisely what led to the discovery of Neptune: its gravitational effects on Uranus. On the other hand, you can't always get away with postulating unseen matter: when Mecury's orbit wasn't behaving right, people tried inventing an unseen planet ("Vulcan"), but it turned out that general relativity was the answer, modifying the laws of gravity. Either can be right a priori.

In the dark matter case, it was once true that the evidence in its favor was strong and there were a number of competing theories, but now there is a lot more evidence, and higher standards for theories, and dark matter is pretty much all that's left. People should and do continue trying to come up with alternatives, but as of now, dark matter is still the best game in town. Far from claims of ad-hockery and epicycles, dark matter is actually a robust physical theory: most theories of dark matter have already been falsified because they make such specific predictions about what we should see. It's only a very specific type, quantity, and distribution of dark matter that can work. That's the hallmark of a good theory, not unfalsifiable wish-fulfillment.

VICTORIA, BRITISH COLUMBIA- When astrophysicists first ran calculations on the observed rotational speeds of nearby galaxies in the 1970's, they ran into a bit of a problem when the numbers didn't add up. According to the familiar laws of Newtonian mechanics, these meta bodies should be much heavier than the number of solar objects would imply. This gap led to one of the most controversial inferences in modern science, thatthe universe contains a massive amount of non-radiating "dark matter" hidden among the stars. For decades scientists were satisfied with this notion. Lectures were delivered, textbooks were printed, and tenure was granted.

A new paper from the University of Victoria, however, casts doubt on all of this. It argues that the whole notion "dark mattter" may be the byproduct of a miscalculation [someone forgot to carry the six] and demonstrates how a proper application of Einstein's principles of general relativity can fully account for a galaxy's rotation and mass without considering this unobserved dark matter. Such contrary ideas often run into resistance, but this theory has engendered far more scientific vitriol than anyone expected.

In fact, when researchers arrived to deliver the paper at a conference last weekend, they were met by an angry mob of civil rights protesters headed by Julian Bond of the NAACP.

"It's fairly clear what's going on here," said Bond through a bullhorn. "Just because it's dark they're saying it doesn't count. I, for one, will not stand for this sort of disenfranchisement. We demand that CERN count all the matter."

Science never has been definite. The defining characteristic of science is that it accepts that all solutions to problems are tenative, and that some piece of information might turn up in the future that will cause us to doubt what we now believe. Intellectual process can't happen without replacing wrong old ideas with better new ones.

It is worth noting that a new idea surpassing current thinking (and demoting current thinking to wrong, old ideas) is not arbitrary. It is not a matter of the old scientists dieing off.

It is a matter of new ideas (a) explaining all of the old observations and experimental results that supported the old theory, as well as (b) explaining observations and experimental results that the old theory could not.

I am not capable of reviewing the observations and redoing the math to verify whether GR by itself explains the observed rotation rates of distant galaxies. Over the next few months more qualified scientists will look at this and publish what they think. Dark matter may go the way of the luminiferous aether. Once it is gone, it is very unlikely to come back in its original form or for its original purpose.

I wonder if this analysis has an effect on the chain of inferences leading to the conclusion that the galactic expansion is accelerating.

Particle physics in particular is an interesting exception to the "old scientists dieing off" rule of how science works. From what I hear, it's commonly accepted that the Standard Model is "wrong", in that the fundamental underlying model is false, but it keeps predicting new observations so well that alternative theories can't get any traction.

"(a) It is worth noting that a new idea surpassing current thinking (and demoting current thinking to wrong, old ideas) is not arbitrary. (b) It is not a matter of the old scientists dieing off."I agree with (a) but disagree with (b). Saying (b) is simply ignoring the social aspects of science because they aren't pretty. But that in fact is how science often works. The new ideas cannot gain traction while the old guard is in control. The new scientists are able to see both theories for what they are, an

The universe is expanding. Has been expanind (in the minds of astronomers, at least) since Hubble's observations let him convince the rest of the astronomers.An interesting question is "is the rate of expansion increasing or decreasing?" GR (even classical Newtonian gravitation) suggests that the expansion rate should be slowing. Observations of Type-I SuperNovae over the last decade or so suggest that they are brighter than we would expect if the expansion were slowing. This brightness is usually taken

On a side note, they are distributing the source. It's possible they may even be GPL friendly.

Note that this is the LaTeX source files for the paper, not source code. What would you do with a GPL scientific paper -- change some things and put your own name on it?

Anyway. I'm surprised it took so long for anyone to do this. Is the an obvious approach, especially if the alternative to postulate entirely new classes of matter. We lesser scientists tend to carry an inferiority complex over the supposed genius of physicists, but I wonder if we've maybe given them too much credit.

The paper only concerns itself with the observed rotation speeds of galaxies, for which "maybe there's something we don't understad about gravity" has always been just as convincing an explanation as dark matter. However, the recent cosmic microwave background radiation [uchicago.edu] data *also* implies dark matter, and doesn't have such an easy alternative explanation. The data tells us that (at least, at the moment the univers first became transparant) baryons only account for 20% or so of mass.

That's not my understanding from the article. I haven't followed galaxy modeling very much, but the article makes the assertion that previous galactic models assumed Newtonian gravity would suffice, since it generally does for distant objects moving at non-relativistic speeds. For example, in our solar system, all of the planets rotations can be explained using Newtonian physics, except for Mercury which is close enough to the sun for the non-linear parts of general relativity to come into effect. So galaxi

Good point. It's also worth noting that even General Relativity doesnt quite get Mercury's orbit right, or the position of those distant probes (Pioneer? the ones past Pluto), so there's clearly something we still don't understand about gravity.

I hate responding to an AC who's unlikely to read the response, but for others who might still be reading, I can summarize what I know. The CMB radiation is a snapshot of the moment in history called "recombination", when the universe changed from a relativistic plasma and became transparant. This happned at about the same instant across the universe because the temperature of the universe was nearly the same at all points.

In a relativistic plasma, a photon doesn't go very far before "hitting" an electron, so the plasma is effectively opaque, but glowing with so much heat that electrons are almost never in a low-energy state, so photons are constantly being re-emitted. The "light pressure" is therefore the dominate force, and the "electron photon soup" acts like a compressible liquid that tries to expand. Over a large enough scale, this is balanced by gravity.

Given we know that the universe was at an extremely uniform temperature, we can predict that it consisted of large cells of gas alternately expanding and contracting. By observing the parrern of temperature differences revealed by the CMB radiation, we can get direct observational evidence about the size and motion of these cells. From our knowledge of plasma physics we can figure the ratio of mass to energy. From the CMB data we can figure the ratio of baryonic mass, which is affected by both light pressure and gravity, and non-baryonic darm matter, which is affected only be gravity.

We actualy have those numbers to about 2 significant digits, which is better than cosmology has ever done in the past with anything. However, the one simplifying assumption in all of this is that the non-baryonic dark matter doesn't interact with light in some strange and complicated way, and while that's the proper assumption to start with, we don't actually know what dark matter is, so who knows.

Have they decided if there is a non-c value for the speed of gravity? I've seen papers concluding yes and I've seen them concluding no.

If gravity has no "speed" then the advisories against instantaneous communication are violated as a change in the relative position of mass A to mass B would instantly be signaled even across the galaxies.

If gravity does have a speed then wouldn't this "dark matter" be explained as all of the extra grativational "signals" making their way through the universe?

If gravity has no "speed" then the advisories against instantaneous communication are violated as a change in the relative position of mass A to mass B would instantly be signaled even across the galaxies.

Which is precisely why such proposals are deeply problematic.

Consider the traditional SR simultaneity paradox -

You have a train, which is carrying a photon torpedo. At time t, klingon saboteurs detonate the torpedo, sending out a pulse of light in all directions. By conventional SR, a viewer on the t

Dark matter isn't even that sensational. Suppose you have equations that would be balanced if you have a certain amount of mass in the universe, and you observe less than that amount. There's two simple explanations: you got the equations wrong, or you're not observing the right amount of mass (in other words, there's some stuff out there we can't see.) Neither idea is that fantastical, and dark matter is just the somewhat catchy name for the stuff we can't see.

I think it's more accurate to say that it is not the scientists, but the pseudo-scientific press that is the problem. The seminal example was the 'Black Hole', a term which the research team neither wanted nor approved of, but which became the name for that phenomenon.

There are some snake-oil sellers out there, but the majority of scientists and researchers roll their eyes when they see the way the general press (and, worse, places like this site) mash theories and garble messages.

Black holes are, well, dark... so all the 'dark' matter is concentrated in localized places, namely the center of the galaxies.

Black holes at the center of galaxies have masses of 10^6 to 10^9 times the mass of the Sun. (Our Galaxy's black hole is towards the smaller side of that range.

Large galaxies themselves have masses of 10^11 to 10^12 times the mass of the Sun.

The black holes at the centers of galaxies, as far as just gravity is concerned, are dynamically unimportant to the outer parts of the galaxies.

Plus, the problem is more than that. It's not just that we don't have enough matter to explain the rotation curves of galaxies or the velocity dispersion of galaxy clusters, it's not in the right place. As you get farther from the center of the galaxy, you need more and more matter compared to what we see. Adding more matter right at the center wouldn't help that, even if the black holes were big enough (which they aren't).

(The black holes may be dynamically important to the evolution of galaxy structure for other more complicated reasons-- the generation of energy in their accretion disks can create jets and such that may limit the growth of galaxies-- but that's a separate issue from expalining the rotation curves we see in spiral galaxies.)

IAAP, and while I see where you're coming from I'd actually make the argument in the opposite direction.

A previous poster has already noted a paper (astro-ph/0508377) which quickly followed this one and refuted its conclusions (I have seen other physicists describe the same point elsewhere). It seems (though I have not yet checked the math myself) that the authors made an honest error, and they weren't modeling the situation they thought they were. In addition to the self-gravitating cloud of gas they were trying to model, the metric also includes a disk-shaped "singularity" - essentially a very thin, very heavy disk in the plane of the galaxy. It is this unphysical disk which is responsible for the effect they observe.

It's also worth noting that dark matter has MANY independent lines of evidence pointing to it (rotation curves, gravitational lensing, the cosmic microwave background, large scale structure, element abundances... see
here [berkeley.edu]). Galactic rotation curves were the first such evidence, but arguably they are the weakest today. I'm still more than willing to believe that the dark matter paradigm could be wrong, and this result would be VERY interesting if true, but there would still be lots left to explain. This is how science works, of course - idea gets put forward, it gets checked by others, the community works out what to think of it.

This also makes me think of the current controversy over intelligent design, but in the opposite way to the previous poster. Look at the Slashdot thread around us. Hundreds of people are posting to say how relieved they are that dark matter doesn't exist, since they always thought it was too weird and that those pointy-headed physicists were out of touch with their own good common sense. They feel very confident doing this, even though (1) they admit that they don't understand the evidence and reasoning they are talking about (even as some of them chastise physicists for the "basic error" they were making), and (2) the reasoning itself was later shown to be flawed. Several posters have tried to make follow-up postings showing that this reasoning has been refuted, but they can't hit every discussion thread (and it's not clear it would do any good if they did). As with the anti-evolution "controversy", people latch on to sensational headlines of flaws in basic science and simplistic errors by scientists to believe whatever they felt most comfortable believing to begin with. From there, it's an uphill battle to get the truth out there.

Dark matter always seemed like it was in the honored high school chemistry tradition of adding a fudge factor. There was o direct observational evidence for it, but tossing it in there made the numbers fit.

But that's how you usually discover things: Make predictions from your current theories, collect data and compare it to the predictions, make up new theories that explain it better, use the data to chose between theories and tell you where to look for more data to make better choices, and iterate.

Sometimes people take shortcuts or make errors in calculation and you have to check their work. And there's valuable science to be done there. But it's more "scientist fun" (and funding) to come up with "George's theory of dark matter" than "George's proof that Sam blew his calculations and Einstien was right after all". So sometimes it takes a while.

Now we wait for "Larry's proof that George blew HIS calculations and Sam was closer to the real world" or "Larry's confirmation that George's model has fewer/smaller holes than Sam's."

Back in the day, the prevailing theory was that the planets were attached to the crystal spheres and travelled in perfect circles. When the data didn't fit, they proposed adding epicycles to the circular paths. When that didn't work, they added more and more circles, increasing the complexity of the theory. Then Copernicus came along and pointed out that it was not so complicated at all... the planets just travelled in ellipses.

Wrong. Copernicus still had the planets moving in circles. The big difference (and the reason of the rejection by the church) was that in his model he put the sun instead of the earth in the center.It was Kepler who found that the planets don't really move in circles, but in ellipses.

I'm in Dr. Cooperstock's General Relativity (GR) class this semester. I must agree, he's a great teacher, and definitely a little quirky. Also he's quite old and some of that quirkiness may come from age.

As an aside, what's different about his lectures is that he uses a transparency roll and an overhead projector instead of the blackboard, and writes/derives everything with us in class, unlike many other professors who merely present slide shows or just talk a lot and write very little (very common among astronomy professors). I really dislike slideshows, and prefer Dr. Cooperstock's method because as he does so, we learn about how he thinks, why he makes the decisions he makes in the derivations, and the usual pitfalls in dealing with all the notation used in GR. That for me is far more valuable than just seeing an amalgamation of details presented on slide shows with a short verbal summary from a professor. Any textbook could provide me with that. The other advantage with the transparency roll is that if we ever need to go back to a previous lecture to revisit something that was discussed there, he just has to put up the roll corresponding to that lecture, and we have it right there in front of us. If we missed any lecture notes, we can just borrow the transparency rolls from him and copy the notes from them.

Back to the topic, I believe that what's important is that we must realize that dark matter is still just a hypothesis. There may be overwhelming signs pointing to something that we call dark matter, but this paper reminds us that dark matter is still only just a hypothesis. It is one of the easiest hypotheses to make, because simply adding a spherical distribution of dark matter to a galactic halo produces the observed rotation curve, but is not the simplest, because it postulates the existence of particles that we cannot yet prove to exist, at least not in such large quantities. If simplicity is a valid reason to accept or refute theories, then Dr. Cooperstock's model appears to me to be simpler because it requires fewer postulates to make things work.

However there are other observations such as satellite galaxies and gravitational lensing and galaxy clusters, all of which appear to require a huge amount of dark matter that we cannot observe. While Dr. Cooperstock's model may not explain all of these yet, this is work that has yet to be done, and so his model cannot be ruled out. One must realize that dark matter is really just a fudge factor to make the theory work out the same way as observations. Until there is good evidence from astronomers and from particle physicists, the arena should be open and impartial to other candidate hypotheses. It is good to see that despite most of the world jumping on to the dark matter band wagon, there are people who stand back and persist with their own ideas. We've seen this happen so many times in history.

Besides, it is still possible that despite GR explaining the galactic rotational curves, dark matter may still exist, but then its role and distribution would change. Oh, the fun of physics...